I have discovered a process that completely saturates the memory on my iMac. It is one of the first 5K iMacs, and it came with 8 GB of memory. I have been happily using it with that much memory for several years. I have beenusing iprocess my photos including HDR processing. But I have gotten a new camera with larger number of pixels and the ability to to auto-bracket five exposures. The first time I tried to do a five exposure HDR in Lightroom my iMac slowed to a crawl. I checked and it was clearly using all available memory.
I bought an additional 16 GB of memory and installed it. I ran my Lightroom workflow, and it worked great. I checked on memory usage, and it was 12 GB. I could have gotten away with just an 8 GB upgrade, but this will provide me with some future proofing.